Wildland fires are a threat in many areas of the world. Effectively controlling and containing these fires is essential to the preservation of national parks, forests, and private property. Significant efforts are therefore directed to developing and improving wildland firefighting techniques.
Wildlands are characterized by difficult and often inaccessible terrain. Fighting fires in such rugged terrain is difficult. In many situations, the only realistic access to burning areas is by air. Therefore, wildland firefighting efforts quite often include the application of water from fixed wing airplanes and helicopters. Water, as it is converted into steam, has tremendous capacity to absorb and carry away heat. It has the further advantage of being environmentally safe. In addition, adequate and convenient reservoirs of water can often be found near burning fires. Helicopters equipped with "slung buckets" can quickly load from such reservoirs without landing. The close proximity of a reservoir also minimizes round trip flying time.
Despite the advantages of water in fighting wildland fires, water alone can be inefficient-especially when applied from the air. Much of the water evaporates before reaching the ground. Water which does reach the ground beads up and rolls off fuels before absorbing its full heat capacity. It has been estimated that water is only 5% to 10% efficient.
Many fire retarding or suppressing chemicals are available which are more efficient than water. However, they are often difficult to use against wildland fires. One problem with such chemicals is that they are environmentally toxic. They are also difficult to transport to remote locations in large quantities. In many cases, fire retardant chemicals require specialized and bulky application equipment. Both the chemicals and the application equipment are expensive.
Foam is one type of fire suppressant which is particularly efficient in terms of both effectiveness and cost. Foam fire suppressant can be created by different methods. However, most methods start with adding a foam concentrate to water in concentrations ranging from 0.1% to 1.0%. PHOS-CHEK.TM. fire suppressant foam concentrate is one popular foam concentrate, available from Monsanto Chemical Company of St. Louis, Mo. One reason for its popularity in fighting wildland fires is its relative biodegradability and its approval by necessary government agencies.
Foam varies from being wet and runny to being as stiff as lather. The stiffness depends on bubble size and expansion ratio. Expansion ratio is the increase in volume as the water becomes foam. Applying foamable liquid under pressure through a standard nozzle results in a relatively low expansion ratio, and a somewhat runny foam. Special air-aspirated nozzles can result in much higher expansion ratios, producing much drier foam.
Foam fire suppressant can be applied from standard fire engines and other vehicles. Foam suppresses fires by surrounding fuel with a thick layer of water, which does not bead up and roll off. This allows the water to absorb its full capacity of heat, and also allows more water to be absorbed into the fuel. Furthermore, a foam blanket creates a vapor barrier between fuel and oncoming fire. It reflects oncoming radiant heat, insulates fuel, continuously releases water from the foam's bubble structure, and helps smother the fuel. A further advantage is that firefighters can easily see where foam has been applied.
Until recently, experts were skeptical of the utility of applying foam to wildland fires from the air. However, this skepticism has now been overcome, and foam concentrate is sometimes added to water applied from aircraft. However, because of the large volume of water which must be quickly dumped to effectively fight wildland fires, applying the water under pressure through nozzles has been impractical. Rather, the water is dumped from conventional slung buckets or fixed tanks. Foam is generated by air turbulence produced by free fall of the water.
The foam created in this manner is relatively runny, having a low expansion ratio. However, the surfactant qualities of the foam concentrate, even without thick foam, are enough of an advantage over water alone to justify the addition of foam concentrate. Nevertheless. it would be desirable to provide thicker foam from airborne applicators.
The most significant obstacle to creating high foam expansion ratios from airborne applicators is the extremely high volume of water which must be applied, at a very high flow rate. The need for a high flow rate is brought about by the speed of the aircraft. In a fixed wing airplane, minimum ground speeds are determined by the very nature of the aircraft. When a helicopter is used to apply water, downdraft or "rotor wash" from the helicopter rotors mandates minimum ground speeds. Helicopter rotor wash is a serious problem for helicopters in wildland fire fighting. It can quickly defeat the effects of applied fire suppressants. The ground turbulence from rotor wash causes the fire to expand, and can endanger nearby fire crews. To minimize rotor wash problems, water applicators are usually suspended a considerable length beneath the helicopter, such as by a distance of fifty feet or more. In addition, minimum ground speeds are maintained to reduce the rotor wash impact on any particular ground area.
The minimum ground speeds required of aircraft when fighting wildland fires mandates a very high application rate of fire suppressant. The flow from a standard firehose nozzle is simply not adequate. However, equipment to produce adequately high application rates of high-expansion foam has not been available for airborne operation. Accordingly, most firefighting efforts continue to rely on standard slung buckets, which depend on gravity to quickly jettison their entire water loads. Foam concentrate is added to the water to act as a surfactant and to provide a minimal amount of foaming through turbulence with the air.
Airborne firefighting effectiveness could be significantly improved if it were possible to apply high-expansion foam from aircraft at application rates approaching those achieved with standard slung buckets. To this date, however, there exists no practical high-expansion foam applicator which will achieve these rates. The invention described below fills the need for such an airborne foam applicator.